The present invention relates to a multilayer liquid crystal display device comprising superposed crystal panel units having plastic film substrates and spacers in a size of not larger than 50 .mu.m dispersed over a surface of a viewing area of each unit.
So far, when a multilayer liquid crystal display device (hereinafter referred to as "LCDD") is fabricated by superposing two or more single liquid crystal panel units (hereinafter referred to as "LCPU") having glass plate substrates, fixing parts, for example, double-sided adhesive tape, of a required thickness are disposed between inner periphery portions of each panel unit to prevent contact between any one the units and a generation of Newton rings, as disclosed, for example, in Japanese Patent Application Laid-Open (KOKAI) No. 54-95,200 (1979).
However, a multilayer LCDD prepared by superposing two or more LCPU having glass plate substrates has defects, namely, (1) large thickness (2) heavy weight and (3) weakness against impact. In order to improved these defects, a multilayer LCDD prepared with a LCPU having plastic film substrates in place of glass plate substrates have been strongly requested. This type of LCDD has many better properties, such as being, thinner, lighter in weight, stronger in impact and much more flexible than the one having glass plate substrates and in view of the improvements, the expectation for the possible realization of a device having a large viewing area and/or a curved surface has increased so far.
The structure and the problems of a LCPU having plastic film substrates (hereinafter referred to as "plastic film LCPU") and a multilayer LCDD prepared by superposing LCPUs are explained referring to FIGS. 1 to 9 attached hereto.
In the Drawings, FIGS. 1 to 5 show a multilayer LCDD of the present invention and FIGS. 6 to 9 show a conventional LCDD prepared with plastic film LCPU.
FIGS. 2 and 6 are a perspective view of a liquid crystal display device having two-layer superposed panel units. FIG. 3 is a cross section view along the line I--I of FIG. 2. FIG. 4 is a cross section view along the III--III of FIG. 2. FIG. 7 is a cross section view along the line V--V of FIG. 6. FIG. 8 illustrates a bad situation caused by a contact of panels 1 and 2. FIG. 9 illustrates a bad appearance in a viewing area of FIG. 8.
Numerals in the Figures denote as follow:
1 a panel unit having spacers,
2 a panel unit not having spacers,
3 a fixing part fixing superposed two units, 1 and 2, and simultaneously maintaining the two units not to contact each other in the viewing areas,
4 an air layer between superposed two units, for example, 1 and 2,
5 a spacer of the present invention,
6 a polarizer,
7 a plastic film substrate,
8 a liquid crystal layer, having particles to maintain clearance between substrates in uniform value (the particles are referred to, hereinafter, as "gap particles"),
9 a sealing material (printed),
A a viewing area.
Each unit has two layers of oriented plastic film substrates 7 provided with transparent electrodes. Gap particles having a size of not more than 10 .mu.m are dispersed on one surface of the one substrate and sealing materials 9 are printed on a peripheral portion of one surface of the other substrate. The surface dispersed with gap particles and the surface printed are stuck together. Liquid crystal is then poured through an inlet prepared in advance into a space between the stuck surface of the substrate and the inlet is then sealed. Polarizers 6 are stuck to each outer surface of the substrates of the intermediately assembled unit and thus completing a LCPU. Two LCPUs 1 and 2 are stuck together with a pair of fixing parts 3, functioning also as spacers, for example, a double-sided adhesion tape, disposed outside of viewing areas A of the surfaces opposite to each other.
If the two-layer LCDD of this type has a size not larger than that of a handhled card type calculator or an IC card, substantially no problem may occur, on the other hand, when the size is larger than about 90 mm .times. about 40 mm, namely, 2-line .times.20-figure in structure, the following problems exist. Under normal conditions, even a conventional LCDD can maintain a constant space clearance between units, shown as an air layer 4 due to existence of the substrates 7, and rigidity of the thick polarizers 6, as shown in FIG. 7. However, when such a device is left under high-temperature and high-humidity conditions, the plastic film LCPUs are deformed due to the difference in expansion and contraction ratios between the substrate and the polarizer and the panel units 1 and 2 are prone to contact partially as shown in FIG. 8.
If an area of the contact is small, Newton rings or interference fringes would be recognized and make an appearance of the device worse.
When the area becomes larger, light scattering through the area contacted, which has no air layer inside, is different from that through the area not contacted. In other words, light transmission is higher at the contacted area than the area not contacted. The presence of areas different in light transmission greatly damages an appearance of the device. FIG. 9 illustrates this bad appearance. The contact region F is higher in light transmission than the non-contact region E. The bad appearance of uneven light transmission can be observed remarkably at driving when panel units 1 and 2 are negative panels.
Although the above description is made based on the panel units 1 and 2 having polarizers 6 on them, the same kind of bad appearance will also occur even when the units have no polarizers, for example, when guest-host liquid crystal units are multilayered. A similar bad appearance will occur also in a LCDD having three or more layer of units.
For this type of device, even if the fixing parts 3 are made thicker somewhat, the panel units 1 and 2 are greatly deformed since the substrates 7 are flexible and the contact occurs at the central portion of the viewing area A and no effect of improvement can be observed by thickening the fixing parts.
To prevent such contact, it is necessary to sufficiently increase the thickness of the fixing parts as a spacer responding to the size of the viewing area. Even in a device having a small viewing area such as 90.times.40 mm, the fixing parts 3 must have a thickness of more than 10 times that of a same size device having glass plate substrates.
This will not only impair the feature that a multilayer LCDD made of plastic film LCPU is thin, but also has no practical value because such a device sometimes is thickner than a display device with a glass plate substrate. Further, in a LCDD having a curved surface it is difficult to maintain the space between the LCPUs only with the fixing parts provided outside the viewing area.
The present inventors have extensively studied to removing the defects explained above, and have an idea to prevent such contact by placing spacers on a viewing area, which has not been able to think of because of the fear that such spacers would damage the uniform light transmission of the device. Accordingly, the inventors studies the size, dispersive density and the light transmission of the spacers deeply and achieved the present invention.